Commissureless acts as a substrate adapter in a conserved Nedd4 E3 ubiquitin ligase pathway to promote axon growth across the midline

In both vertebrates and invertebrates, commissural neurons prevent premature responsiveness to the midline repellant Slit by downregulating surface levels of its receptor Roundabout1 (Robo1). In Drosophila, Commissureless (Comm) plays a critical role in this process; however, there is conflicting data on the underlying molecular mechanism. Here, we demonstrate that the conserved PY motifs in the cytoplasmic domain of Comm are required allow the ubiquitination and lysosomal degradation of Robo1. Disruption of these motifs prevents Comm from localizing to Lamp1 positive late endosomes and to promote axon growth across the midline in vivo. In addition, we conclusively demonstrate a role for Nedd4 in midline crossing. Genetic analysis shows that nedd4 mutations result in midline crossing defects in the Drosophila embryonic nerve cord, which can be rescued by introduction of exogenous Nedd4. Biochemical evidence shows that Nedd4 incorporates into a three-member complex with Comm and Robo in a PY motif-dependent manner. Finally, we present genetic evidence that Nedd4 acts with Comm in the embryonic nerve cord to downregulate Robo1 levels. Taken together, these findings demonstrate that Comm promotes midline crossing in the nerve cord by facilitating Robo ubiquitination by Nedd4, ultimately leading to its degradation.

Systematic analysis of the Frazzled receptor interactome establishes previously unreported regulators of axon guidance

The Netrin receptor Dcc and its Drosophila homolog Frazzled play crucial roles in diverse developmental process, including axon guidance. In Drosophila, Fra regulates midline axon guidance through a Netrin-dependent and a Netrin-independent pathway. However, what molecules regulate these distinct signaling pathways remain unclear. To identify Fra-interacting proteins, we performed affinity purification mass spectrometry to establish a neuronal-specific Fra interactome. In addition to known interactors of Fra and Dcc, including Netrin and Robo1, our screen identified 85 candidate proteins, the majority of which are conserved in humans. Many of these proteins are expressed in the ventral nerve cord, and gene ontology, pathway analysis and biochemical validation identified several previously unreported pathways, including the receptor tyrosine phosphatase Lar, subunits of the COP9 signalosome and Rho-5, a regulator of the metalloprotease Tace. Finally, genetic analysis demonstrates that these genes regulate axon guidance and may define as yet unknown signaling mechanisms for Fra and its vertebrate homolog Dcc. Thus, the Fra interactome represents a resource to guide future functional studies.

DrosoPHILA: A Partnership between Scientists and Teachers That Begins in the Lab and Continues into City Schools

Here, we describe the development, structure, and effectiveness of an outreach program, DrosoPHILA, that leverages the tools of our fly neurodevelopmental research program at the University of Pennsylvania to reinforce the biology curriculum in local public schools. DrosoPHILA was developed and is sustained by a continued collaboration between members of the Bashaw lab, experienced outreach educators, and teachers in the School District of Philadelphia. Since the program's inception, we have collaborated with 18 teachers and over 2400 students. Student outcome data indicates significant positive attitude shifts around science identity and grade-appropriate knowledge gains.

Intracellular Trafficking Mechanisms that Regulate Repulsive Axon Guidance

Friedrich Bonhoeffer made seminal contributions to the study of axon guidance in the developing nervous system. His discoveries of key cellular and molecular mechanisms that dictate wiring specificity laid the foundation for countless investigators who have followed in his footsteps. Perhaps his most significant contribution was the cloning and characterization of members of the conserved ephrin family of repulsive axon guidance cues. In this review, we highlight the major contributions that Bonhoeffer and his colleagues made to the field of axon guidance, and discuss ongoing investigations into the diverse array of mechanisms that ensure that axon repulsion is precisely regulated to allow for accurate pathfinding. Specifically, we focus our discussion on the post-translational regulation of two major families of repulsive axon guidance factors: ephrin ligands and their Eph receptors, and slit ligands and their Roundabout (Robo) receptors. We will give special emphasis to the ways in which regulated endocytic trafficking events allow navigating axons to adjust their responses to repellant signals and how these trafficking events are intimately related to receptor signaling. By highlighting parallels and differences between the regulation of these two important repulsive axon guidance pathways, we hope to identify key outstanding questions for future investigation.

Tace/ADAM17 is a bi-directional regulator of axon guidance that coordinates distinct Frazzled and Dcc receptor signaling outputs

Frazzled (Fra) and deleted in colorectal cancer (Dcc) are homologous receptors that promote axon attraction in response to netrin. In Drosophila, Fra also acts independently of netrin by releasing an intracellular domain (ICD) that activates gene transcription. How neurons coordinate these pathways to make accurate guidance decisions is unclear. Here we show that the ADAM metalloprotease Tace cleaves Fra, and this instructs the switch between the two pathways. Genetic manipulations that either increase or decrease Tace levels disrupt midline crossing of commissural axons. These conflicting phenotypes reflect Tace's function as a bi-directional regulator of axon guidance, a function conserved in its vertebrate homolog ADAM17: while Tace induces the formation of the Fra ICD to activate transcription, excessive Tace cleavage of Fra and Dcc suppresses the response to netrin. We propose that Tace and ADAM17 are key regulators of midline axon guidance by establishing the balance between netrin-dependent and netrin-independent signaling.

Diverse roles for axon guidance pathways in adult tissue architecture and function

Classical axon guidance ligands and their neuronal receptors were first identified due to their fundamental roles in regulating connectivity in the developing nervous system. Since their initial discovery, it has become clear that these signaling molecules play important roles in the development of a broad array of tissue and organ systems across phylogeny. In addition to these diverse developmental roles, there is a growing appreciation that guidance signaling pathways have important functions in adult organisms, including the regulation of tissue integrity and homeostasis. These roles in adult organisms include both tissue-intrinsic activities of guidance molecules, as well as systemic effects on tissue maintenance and function mediated by the nervous and vascular systems. While many of these adult functions depend on mechanisms that mirror developmental activities, such as regulating adhesion and cell motility, there are also examples of adult roles that may reflect signaling activities that are distinct from known developmental mechanisms, including the contributions of guidance signaling pathways to lineage commitment in the intestinal epithelium and bone remodeling in vertebrates. In this review, we highlight studies of guidance receptors and their ligands in adult tissues outside of the nervous system, focusing on in vivo experimental contexts. Together, these studies lay the groundwork for future investigation into the conserved and tissue-specific mechanisms of guidance receptor signaling in adult tissues.

Frazzled/Dcc acts independently of Netrin to promote germline survival during Drosophila oogenesis

The Netrin receptor Frazzled/Dcc (Fra in Drosophila) functions in diverse tissue contexts to regulate cell migration, axon guidance and cell survival. Fra signals in response to Netrin to regulate the cytoskeleton and also acts independently of Netrin to directly regulate transcription during axon guidance in Drosophila. In other contexts, Dcc acts as a tumor suppressor by directly promoting apoptosis. In this study, we report that Fra is required in the Drosophila female germline for the progression of egg chambers through mid-oogenesis. Loss of Fra in the germline, but not the somatic cells of the ovary, results in the degeneration of egg chambers. Although a failure in nutrient sensing and disruptions in egg chamber polarity can result in degeneration at mid-oogenesis, these factors do not appear to be affected in fra germline mutants. However, similar to the degeneration that occurs in those contexts, the cell death effector Dcp-1 is activated in fra germline mutants. The function of Fra in the female germline is independent of Netrin and requires the transcriptional activation domain of Fra. In contrast to the role of Dcc in promoting cell death, our observations reveal a role for Fra in regulating germline survival by inhibiting apoptosis.

Robo recruitment of the Wave regulatory complex plays an essential and conserved role in midline repulsion

The Roundabout (Robo) guidance receptor family induces axon repulsion in response to its ligand Slit by inducing local cytoskeletal changes; however, the link to the cytoskeleton and the nature of these cytoskeletal changes are poorly understood. Here, we show that the heteropentameric Scar/Wave Regulatory Complex (WRC), which drives Arp2/3-induced branched actin polymerization, is a direct effector of Robo signaling. Biochemical evidence shows that Slit triggers WRC recruitment to the Robo receptor's WRC-interacting receptor sequence (WIRS) motif. In Drosophila embryos, mutants of the WRC enhance Robo1-dependent midline crossing defects. Additionally, mutating Robo1's WIRS motif significantly reduces receptor activity in rescue assays in vivo, and CRISPR-Cas9 mutagenesis shows that the WIRS motif is essential for endogenous Robo1 function. Finally, axon guidance assays in mouse dorsal spinal commissural axons and gain-of-function experiments in chick embryos demonstrate that the WIRS motif is also required for Robo1 repulsion in mammals. Together, our data support an essential conserved role for the WIRS-WRC interaction in Robo1-mediated axon repulsion.

Conserved and divergent aspects of Robo receptor signaling and regulation between Drosophila Robo1 and C. elegans SAX-3

The evolutionarily conserved Roundabout (Robo) family of axon guidance receptors control midline crossing of axons in response to the midline repellant ligand Slit in bilaterian animals including insects, nematodes, and vertebrates. Despite this strong evolutionary conservation, it is unclear whether the signaling mechanism(s) downstream of Robo receptors are similarly conserved. To directly compare midline repulsive signaling in Robo family members from different species, here we use a transgenic approach to express the Robo family receptor SAX-3 from the nematode Caenorhabditis elegans in neurons of the fruit fly, Drosophila melanogaster. We examine SAX-3's ability to repel Drosophila axons from the Slit-expressing midline in gain of function assays, and test SAX-3's ability to substitute for Drosophila Robo1 during fly embryonic development in genetic rescue experiments. We show that C. elegans SAX-3 is properly translated and localized to neuronal axons when expressed in the Drosophila embryonic CNS, and that SAX-3 can signal midline repulsion in Drosophila embryonic neurons, although not as efficiently as Drosophila Robo1. Using a series of Robo1/SAX-3 chimeras, we show that the SAX-3 cytoplasmic domain can signal midline repulsion to the same extent as Robo1 when combined with the Robo1 ectodomain. We show that SAX-3 is not subject to endosomal sorting by the negative regulator Commissureless (Comm) in Drosophila neurons in vivo, and that peri-membrane and ectodomain sequences are both required for Comm sorting of Drosophila Robo1.

New insights into the molecular mechanisms of axon guidance receptor regulation and signaling

As the nervous system develops, newly differentiated neurons need to extend their axons toward their synaptic targets to form functional neural circuits. During this highly dynamic process of axon pathfinding, guidance receptors expressed at the tips of motile axons interact with soluble guidance cues or membrane tethered molecules present in the environment to be either attracted toward or repelled away from the source of these cues. As competing cues are often present at the same location and during the same developmental period, guidance receptors need to be both spatially and temporally regulated in order for the navigating axons to make appropriate guidance decisions. This regulation is exerted by a diverse array of molecular mechanisms that have come into focus over the past several decades and these mechanisms ensure that the correct complement of surface receptors is present on the growth cone, a fan-shaped expansion at the tip of the axon. This dynamic, highly motile structure is defined by a lamellipodial network lining the periphery of the growth cone interspersed with finger-like filopodial projections that serve to explore the surrounding environment. Once axon guidance receptors are deployed at the right place and time at the growth cone surface, they respond to their respective ligands by initiating a complex set of signaling events that serve to rearrange the growth cone membrane and the actin and microtubule cytoskeleton to affect axon growth and guidance. In this review, we highlight recent advances that shed light on the rich complexity of mechanisms that regulate axon guidance receptor distribution, activation and downstream signaling.

Molecular mechanisms regulating axon responsiveness at the midline

During embryonic development in bilaterally symmetric organisms, correct midline crossing is important for the proper formation of functional neural circuits. The aberrant development of neural circuits can result in multiple neurodevelopmental disorders, including horizontal gaze palsy, congenital mirror movement disorder, and autism spectrum disorder. Thus, understanding the molecular mechanisms that regulate proper axon guidance at the midline can provide insights into the pathology of neurological disorders. The signaling mechanisms that regulate midline crossing have been extensively studied in the Drosophila ventral nerve cord and the mouse embryonic spinal cord. In this review, we discuss these axon guidance mechanisms, highlighting the most recent advances in the understanding of how commissural axons switch their responsiveness from attractants to repellents during midline crossing.

Ndfip Proteins Target Robo Receptors for Degradation and Allow Commissural Axons to Cross the Midline in the Developing Spinal Cord

Commissural axons initially respond to attractive signals at the midline, but once they cross, they become sensitive to repulsive cues. This switch prevents axons from re-entering the midline. In insects and mammals, negative regulation of Roundabout (Robo) receptors prevents premature response to the midline repellant Slit. In Drosophila, the endosomal protein Commissureless (Comm) prevents Robo1 surface expression before midline crossing by diverting Robo1 into late endosomes. Notably, Comm is not conserved in vertebrates. We identified two Nedd-4-interacting proteins, Ndfip1 and Ndfip2, that act analogously to Comm to localize Robo1 to endosomes. Ndfip proteins recruit Nedd4 E3 ubiquitin ligases to promote Robo1 ubiquitylation and degradation. Ndfip proteins are expressed in commissural axons in the developing spinal cord and removal of Ndfip proteins results in increased Robo1 expression and reduced midline crossing. Our results define a conserved Robo1 intracellular sorting mechanism between flies and mammals to avoid premature responsiveness to Slit.

Brain Tumor promotes axon growth across the midline through interactions with the microtubule stabilizing protein Apc2

Commissural axons must cross the midline to establish reciprocal connections between the two sides of the body. This process is highly conserved between invertebrates and vertebrates and depends on guidance cues and their receptors to instruct axon trajectories. The DCC family receptor Frazzled (Fra) signals chemoattraction and promotes midline crossing in response to its ligand Netrin. However, in Netrin or fra mutants, the loss of crossing is incomplete, suggesting the existence of additional pathways. Here, we identify Brain Tumor (Brat), a tripartite motif protein, as a new regulator of midline crossing in the Drosophila CNS. Genetic analysis indicates that Brat acts independently of the Netrin/Fra pathway. In addition, we show that through its B-Box domains, Brat acts cell autonomously to regulate the expression and localization of Adenomatous polyposis coli-2 (Apc2), a key component of the Wnt canonical signaling pathway, to promote axon growth across the midline. Genetic evidence indicates that the role of Brat and Apc2 to promote axon growth across the midline is independent of Wnt and Beta-catenin-mediated transcriptional regulation. Instead, we propose that Brat promotes midline crossing through directing the localization or stability of Apc2 at the plus ends of microtubules in navigating commissural axons. These findings define a new mechanism in the coordination of axon growth and guidance at the midline.

The establishment of neuronal connections that cross the midline of the animal is essential to generate neural circuits that coordinate the left and right sides of the body. Axons that cross the midline to form these connections are called commissural axons and the molecules and mechanisms that control midline axon crossing are remarkably conserved across animal evolution. In this study we have used a genetic screen in the fruit fly in an attempt to uncover additional players in this key developmental process, and have identified a novel role for the Brain Tumor (Brat) protein in promoting commissural axon growth across the midline. Unlike its previous described functions, in the context of midline axon guidance Brat cooperates with the microtubule stabilizing protein Apc2 to coordinate axon growth and guidance. Molecular and genetic analyses point to the conserved B box motifs of the Brat protein as key in promoting the association of Apc2 with the plus ends of microtubules. Brat is highly conserved and future studies will determine whether homologous genes play analogous roles in mammalian neural development.

Axon guidance pathways and the control of gene expression

Axons need to be properly guided to their targets to form synaptic connections, and this requires interactions between highly conserved extracellular and transmembrane ligands and their cell surface receptors. The majority of studies on axon guidance signaling pathways have focused on the role of these pathways in rearranging the local cytoskeleton and plasma membrane in growth cones and axons. However, a smaller body of work has demonstrated that axon guidance signaling pathways also control gene expression via local translation and transcription. Recent studies on axon guidance ligands and receptors have begun to uncover the requirements for these alternative mechanisms in processes required for neural circuit formation: axon guidance, synaptogenesis, and cell migration. Understanding the mechanisms by which axon guidance signaling regulates local translation and transcription will create a more complete picture of neural circuit formation, and they may be applied more broadly to other tissues where axon guidance ligands and receptors are required for morphogenesis. Developmental Dynamics 247:571-580, 2018. © 2017 Wiley Periodicals, Inc.

Islet Coordinately Regulates Motor Axon Guidance and Dendrite Targeting through the Frazzled/DCC Receptor

Motor neuron axon targeting in the periphery is correlated with the positions of motor neuron inputs in the CNS, but how these processes are coordinated to form a myotopic map remains poorly understood. We show that the LIM homeodomain factor Islet (Isl) controls targeting of both axons and dendrites in Drosophila motor neurons through regulation of the Frazzled (Fra)/DCC receptor. Isl is required for fra expression in ventrally projecting motor neurons, and isl and fra mutants have similar axon guidance defects. Single-cell labeling indicates that isl and fra are also required for dendrite targeting in a subset of motor neurons. Finally, overexpression of Fra rescues axon and dendrite targeting defects in isl mutants. These results indicate that Fra acts downstream of Isl in both the periphery and the CNS, demonstrating how a single regulatory relationship is used in multiple cellular compartments to coordinate neural circuit wiring.

Sema-1a Reverse Signaling Promotes Midline Crossing in Response to Secreted Semaphorins

Commissural axons must cross the midline to form functional midline circuits. In the invertebrate nerve cord and vertebrate spinal cord, midline crossing is mediated in part by Netrin-dependent chemoattraction. Loss of crossing, however, is incomplete in mutants for Netrin or its receptor Frazzled/DCC, suggesting the existence of additional pathways. We identified the transmembrane Semaphorin, Sema-1a, as an important regulator of midline crossing in the Drosophila CNS. We show that in response to the secreted Semaphorins Sema-2a and Sema-2b, Sema-1a functions as a receptor to promote crossing independently of Netrin. In contrast to other examples of reverse signaling where Sema1a triggers repulsion through activation of Rho in response to Plexin binding, in commissural neurons Sema-1a acts independently of Plexins to inhibit Rho to promote attraction to the midline. These findings suggest that Sema-1a reverse signaling can elicit distinct axonal responses depending on differential engagement of distinct ligands and signaling effectors.

The Intracellular Domain of the Frazzled/DCC Receptor Is a Transcription Factor Required for Commissural Axon Guidance

In commissural neurons of Drosophila, the conserved Frazzled (Fra)/Deleted in Colorectal Cancer (DCC) receptor promotes midline axon crossing by signaling locally in response to Netrin and by inducing transcription of commissureless (comm), an antagonist of Slit-Roundabout midline repulsion, through an unknown mechanism. Here, we show that Fra is cleaved to release its intracellular domain (ICD), which shuttles between the cytoplasm and the nucleus, where it functions as a transcriptional activator. Rescue and gain-of-function experiments demonstrate that the Fra ICD is sufficient to regulate comm expression and that both γ-secretase proteolysis of Fra and Fra's function as a transcriptional activator are required for its ability to regulate comm in vivo. Our data uncover an unexpected role for the Fra ICD as a transcription factor whose activity regulates the responsiveness of commissural axons at the midline and raise the possibility that nuclear signaling may be a common output of axon guidance receptors.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

Robo2 acts in trans to inhibit Slit-Robo1 repulsion in pre-crossing commissural axons

During nervous system development, commissural axons cross the midline despite the presence of repellant ligands. In Drosophila, commissural axons avoid premature responsiveness to the midline repellant Slit by expressing the endosomal sorting receptor Commissureless, which reduces surface expression of the Slit receptor Roundabout1 (Robo1). In this study, we describe a distinct mechanism to inhibit Robo1 repulsion and promote midline crossing, in which Roundabout2 (Robo2) binds to and prevents Robo1 signaling. Unexpectedly, we find that Robo2 is expressed in midline cells during the early stages of commissural axon guidance, and that over-expression of Robo2 can rescue robo2-dependent midline crossing defects non-cell autonomously. We show that the extracellular domains required for binding to Robo1 are also required for Robo2's ability to promote midline crossing, in both gain-of-function and rescue assays. These findings indicate that at least two independent mechanisms to overcome Slit-Robo1 repulsion in pre-crossing commissural axons have evolved in Drosophila.

Crossing the embryonic midline: molecular mechanisms regulating axon responsiveness at an intermediate target

In bilaterally symmetric animals, the precise assembly of neural circuitry at the midline is essential for coordination of the left and right sides of the body. Commissural axons must first be directed across the midline and then be prevented from re-crossing in order to ensure proper midline connectivity. Here, we review the attractants and repellents that direct axonal navigation at the ventral midline and the receptors on commissural neurons through which they signal. In addition, we discuss the mechanisms that commissural axons use to switch their responsiveness to midline-derived cues, so that they are initially responsive to midline attractants and subsequently responsive to midline repellents.

Transcription factors and effectors that regulate neuronal morphology

Transcription factors establish the tremendous diversity of cell types in the nervous system by regulating the expression of genes that give a cell its morphological and functional properties. Although many studies have identified requirements for specific transcription factors during the different steps of neural circuit assembly, few have identified the downstream effectors by which they control neuronal morphology. In this Review, we highlight recent work that has elucidated the functional relationships between transcription factors and the downstream effectors through which they regulate neural connectivity in multiple model systems, with a focus on axon guidance and dendrite morphogenesis.

The homeodomain transcription factor Hb9 controls axon guidance in Drosophila through the regulation of Robo receptors

Transcription factors establish neural diversity and wiring specificity; however, how they orchestrate changes in cell morphology remains poorly understood. The Drosophila Roundabout (Robo) receptors regulate connectivity in the CNS, but how their precise expression domains are established is unknown. Here, we show that the homeodomain transcription factor Hb9 acts upstream of Robo2 and Robo3 to regulate axon guidance in the Drosophila embryo. In ventrally projecting motor neurons, hb9 is required for robo2 expression, and restoring Robo2 activity in hb9 mutants rescues motor axon defects. Hb9 requires its conserved repressor domain and functions in parallel with Nkx6 to regulate robo2. Moreover, hb9 can regulate the medio-lateral position of axons through robo2 and robo3, and restoring robo3 expression in hb9 mutants rescues the lateral position defects of a subset of neurons. Altogether, these data identify Robo2 and Robo3 as key effectors of Hb9 in regulating nervous system development.

A transcription factor network coordinates attraction, repulsion, and adhesion combinatorially to control motor axon pathway selection

Combinations of transcription factors (TFs) instruct precise wiring patterns in the developing nervous system; however, how these factors impinge on surface molecules that control guidance decisions is poorly understood. Using mRNA profiling, we identified the complement of membrane molecules regulated by the homeobox TF Even-skipped (Eve), the major determinant of dorsal motor neuron (dMN) identity in Drosophila. Combinatorial loss- and gain-of-function genetic analyses of Eve target genes indicate that the integrated actions of attractive, repulsive, and adhesive molecules direct eve-dependent dMN axon guidance. Furthermore, combined misexpression of Eve target genes is sufficient to partially restore CNS exit and can convert the guidance behavior of interneurons to that of dMNs. Finally, we show that a network of TFs, comprised of eve, zfh1, and grain, induces the expression of the Unc5 and Beaten-path guidance receptors and the Fasciclin 2 and Neuroglian adhesion molecules to guide individual dMN axons.

Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

To develop a functional nervous system, axons must initially navigate through a complex environment, directed by guidance ligands and receptors. These receptors must link to intracellular signaling cascades to direct axon pathfinding decisions. The Abelson tyrosine kinase (Abl) plays a crucial role in multiple Drosophila axon guidance pathways during development, though the mechanism by which Abl elicits a diverse set of guidance outputs is currently unknown. We identified Abl in a genetic screen for genes that contribute to Netrin-dependent axon guidance in midline-crossing (commissural) neurons. We find that Abl interacts both physically and genetically with the Netrin receptor Frazzled, and that disrupting this interaction prevents Abl from promoting midline axon crossing. Moreover, we find that Abl exerts its diverse activities through at least two different mechanisms: (1) a partly kinase-independent, structural function in midline attraction through its C-terminal F-actin binding domain (FABD) and (2) a kinase-dependent inhibition of repulsive guidance pathways that does not require the Abl C terminus. Abl also regulates motor axon pathfinding through a non-overlapping set of functional domains. These results highlight how a multifunctional kinase can trigger diverse axon guidance outcomes through the use of distinct structural motifs.

Slit/Robo-mediated axon guidance in Tribolium and Drosophila: divergent genetic programs build insect nervous systems

As the complexity of animal nervous systems has increased during evolution, developmental control of neuronal connectivity has become increasingly refined. How has functional diversification within related axon guidance molecules contributed to the evolution of nervous systems? To address this question, we explore the evolution of functional diversity within the Roundabout (Robo) family of axon guidance receptors. In Drosophila, Robo and Robo2 promote midline repulsion, while Robo2 and Robo3 specify the position of longitudinal axon pathways. The Robo family has expanded by gene duplication in insects; robo2 and robo3 exist as distinct genes only within dipterans, while other insects, like the flour beetle Tribolium castaneum, retain an ancestral robo2/3 gene. Both Robos from Tribolium can mediate midline repulsion in Drosophila, but unlike the fly Robos cannot be down-regulated by Commissureless. The overall architecture and arrangement of longitudinal pathways are remarkably conserved in Tribolium, despite it having only two Robos. Loss of TcSlit causes midline collapse of axons in the beetle, a phenotype recapitulated by simultaneous knockdown of both Robos. Single gene knockdowns reveal that beetle Robos have specialized axon guidance functions: TcRobo is dedicated to midline repulsion, while TcRobo2/3 also regulates longitudinal pathway formation. TcRobo2/3 knockdown reproduces aspects of both Drosophila robo2 and robo3 mutants, suggesting that TcRobo2/3 has two functions that in Drosophila are divided between Robo2 and Robo3. The ability of Tribolium to organize longitudinal axons into three discrete medial-lateral zones with only two Robo receptors demonstrates that beetle and fly achieve equivalent developmental outcomes using divergent genetic programs.

Live dissection and surface labeling of proteins in Drosophila embryos

This protocol presents methods for the specific detection of neuronal cell-surface receptors and secreted factors. These techniques allow visualization of surface pools of receptor proteins and also reveal the surface distribution of secreted cues. Live dissection of embryos to expose the ventral nerve cord is also described. This procedure can be used to improve the ability to detect signals from suboptimal antibodies, where conventional staining gives poor results.

Visualizing axons in the Drosophila central nervous system using immunohistochemistry and immunofluorescence

How axons in the developing nervous system navigate to their correct targets is a fundamental question in neuroscience. Studies of axon guidance in the embryonic central nervous system (CNS) of Drosophila melanogaster have proved instrumental to the identification of molecules and mechanisms that regulate wiring. The relative simplicity of the Drosophila embryonic CNS, the advantages of genetic approaches, and the ability to analyze the pathfinding decisions of well-defined axon groups or even individual axons have all contributed to our understanding of the fundamental mechanisms of axon guidance and target selection. In this protocol, we describe methods for using fluorescent antibody staining as well as horseradish peroxidase (HRP) immunohistochemistry to reveal axon-projection patterns and identify specific guidance molecules. In the presence of 3,3'-diaminobenzidine tetrahydrochloride (DAB), HRP forms a brown, stable precipitate that allows visualization of axon populations under white light. This can be quite advantageous when teaching basic embryonic anatomy and dissection techniques, because unlike fluorescent antibody staining, HRP can be visualized in a lighted room. HRP detection does not allow precise colocalization of signals, nor can multiple colors be visualized simultaneously. These objectives are easily achieved with fluorescent antibody staining.

The Adam family metalloprotease Kuzbanian regulates the cleavage of the roundabout receptor to control axon repulsion at the midline

Slits and their Roundabout (Robo) receptors mediate repulsive axon guidance at the Drosophila ventral midline and in the vertebrate spinal cord. Slit is cleaved to produce fragments with distinct signaling properties. In a screen for genes involved in Slit-Robo repulsion, we have identified the Adam family metalloprotease Kuzbanian (Kuz). Kuz does not regulate midline repulsion through cleavage of Slit, nor is Slit cleavage essential for repulsion. Instead, Kuz acts in neurons to regulate repulsion and Kuz can cleave the Robo extracellular domain in Drosophila cells. Genetic rescue experiments using an uncleavable form of Robo show that this receptor does not maintain normal repellent activity. Finally, Kuz activity is required for Robo to recruit its downstream signaling partner, Son of sevenless (Sos). These observations support the model that Kuz-directed cleavage is important for Robo receptor activation.

Signaling from axon guidance receptors

Determining how axon guidance receptors transmit signals to allow precise pathfinding decisions is fundamental to our understanding of nervous system development and may suggest new strategies to promote axon regeneration after injury or disease. Signaling mechanisms that act downstream of four prominent families of axon guidance cues--netrins, semaphorins, ephrins, and slits--have been extensively studied in both invertebrate and vertebrate model systems. Although details of these signaling mechanisms are still fragmentary and there appears to be considerable diversity in how different guidance receptors regulate the motility of the axonal growth cone, a number of common themes have emerged. Here, we review recent insights into how specific receptors for each of these guidance cues engage downstream regulators of the growth cone cytoskeleton to control axon guidance.

Functional diversity of Robo receptor immunoglobulin domains promotes distinct axon guidance decisions

Recognition molecules of the immunoglobulin (Ig) superfamily control axon guidance in the developing nervous system. Ig-like domains are among the most widely represented protein domains in the human genome, and the number of Ig superfamily proteins is strongly correlated with cellular complexity. In Drosophila, three Roundabout (Robo) Ig superfamily receptors respond to their common Slit ligand to regulate axon guidance at the midline: Robo and Robo2 mediate midline repulsion, Robo2 and Robo3 control longitudinal pathway selection, and Robo2 can promote midline crossing. How these closely related receptors mediate distinct guidance functions is not understood. We report that the differential functions of Robo2 and Robo3 are specified by their ectodomains and do not reflect differences in cytoplasmic signaling. Functional modularity of Robo2's ectodomain facilitates multiple guidance decisions: Ig1 and Ig3 of Robo2 confer lateral positioning activity, whereas Ig2 confers promidline crossing activity. Robo2's distinct functions are not dependent on greater Slit affinity but are instead due in part to differences in multimerization and receptor-ligand stoichiometry conferred by Robo2's Ig domains. Together, our findings suggest that diverse responses to the Slit guidance cue are imparted by intrinsic structural differences encoded in the extracellular Ig domains of the Robo receptors.

Axon growth and guidance: receptor regulation and signal transduction

The development of precise connectivity patterns during the establishment of the nervous system depends on the regulated action of diverse, conserved families of guidance cues and their neuronal receptors. Determining how these signaling pathways function to regulate axon growth and guidance is fundamentally important to understanding wiring specificity in the nervous system and will undoubtedly shed light on many neural developmental disorders. Considerable progress has been made in defining the mechanisms that regulate the correct spatial and temporal distribution of guidance receptors and how these receptors in turn signal to the growth cone cytoskeleton to control steering decisions. This review focuses on recent advances in our understanding of the mechanisms mediating growth cone guidance with a particular emphasis on the control of guidance receptor regulation and signaling.

A frazzled/DCC-dependent transcriptional switch regulates midline axon guidance

Precise wiring of the nervous system depends on coordinating the action of conserved families of proteins that direct axons to their appropriate targets. Slit-roundabout repulsion and netrin-deleted in colorectal cancer (DCC) (frazzled) attraction must be tightly regulated to control midline axon guidance in vertebrates and invertebrates, but the mechanism mediating this regulation is poorly defined. Here, we show that the Fra receptor has two genetically separable functions in regulating midline guidance in Drosophila. First, Fra mediates canonical chemoattraction in response to netrin, and, second, it functions independently of netrin to activate commissureless transcription, allowing attraction to be coupled to the down-regulation of repulsion in precrossing commissural axons.

Cytoplasmic domain requirements for Frazzled-mediated attractive axon turning at the Drosophila midline

The conserved DCC ligand-receptor pair Netrin and Frazzled (Fra) has a well-established role in axon guidance. However, the specific sequence motifs required for orchestrating downstream signaling events are not well understood. Evidence from vertebrates suggests that P3 is important for transducing Netrin-mediated turning and outgrowth, whereas in C. elegans it was shown that the P1 and P2 conserved sequence motifs are required for a gain-of-function outgrowth response. Here, we demonstrate that Drosophila fra mutant embryos exhibit guidance defects in a specific subset of commissural axons and these defects can be rescued cell-autonomously by expressing wild-type Fra exclusively in these neurons. Furthermore, structure-function studies indicate that the conserved P3 motif (but not P1 or P2) is required for growth cone attraction at the Drosophila midline. Surprisingly, in contrast to vertebrate DCC, P3 does not mediate receptor self-association, and self-association is not sufficient to promote Fra-dependent attraction. We also show that in contrast to previous findings, the cytoplasmic domain of Fra is not required for axonal localization and that neuronal expression of a truncated Fra receptor lacking the entire cytoplasmic domain (Fra delta C) results in dose-dependent defects in commissural axon guidance. These findings represent the first systematic dissection of the cytoplasmic domains required for Fra-mediated axon attraction in the context of full-length receptors in an intact organism and provide important insights into attractive axon guidance at the midline.

Independent functions of Slit-Robo repulsion and Netrin-Frazzled attraction regulate axon crossing at the midline in Drosophila

Slit and Netrin and their respective neuronal receptors play critical roles in patterning axonal connections in the developing nervous system by regulating the decision of whether or not to cross the midline. Studies of both invertebrate and vertebrate systems support the idea that Netrin, secreted by midline cells, signals through DCC (Deleted in Colorectal Carcinoma)/UNC40/Frazzled receptors to attract commissural axons toward and across the midline, whereas Slit signals through Robo family receptors to prevent commissural axons from recrossing the midline, as well as to prevent ipsilateral axons from ever crossing. Recent evidence from both Xenopus neuronal cell culture and Drosophila genetics have suggested that these signals may interact more directly in a hierarchical relationship, such that one response extinguishes the other. Here we present loss- and gain-of-function genetic evidence showing that the influence of Slit and Netrin on midline axon crossing is dictated by both independent and interdependent signaling functions of the Robo and Frazzled (Fra) receptors. Our results are not consistent with the proposal based on genetic analysis in Drosophila that the sole function of Slit and Robo during midline guidance is to repress Netrin attraction.

Semaphorin directs axon traffic in the fly olfactory system

The convergence of olfactory sensory axons that express the same receptor onto specific glomeruli is a common organizing principle in animal olfactory systems. In this issue of Neuron, two beautiful studies in Drosophila by Lattemann et al. and Sweeney et al. show that Semaphorin repulsion regulates interactions between olfactory receptor neurons to help axons find their correct targets.

beta-Spectrin functions independently of Ankyrin to regulate the establishment and maintenance of axon connections in the Drosophila embryonic CNS

alpha- and beta-Spectrin are major components of a submembrane cytoskeletal network connecting actin filaments to integral plasma membrane proteins. Besides its structural role in red blood cells, the Spectrin network is thought to function in non-erythroid cells during protein targeting and membrane domain formation. Here, we demonstrate that beta-Spectrin is required in neurons for proper midline axon guidance in the Drosophila embryonic CNS. In beta-spectrin mutants many axons inappropriately cross the CNS midline, suggesting a role for beta-Spectrin in midline repulsion. Surprisingly, neither the Ankyrin-binding nor the pleckstrin homology (PH) domains of beta-Spectrin are required for accurate guidance decisions. alpha-Spectrin is dependent upon beta-Spectrin for its normal subcellular localization and/or maintenance, whereas alpha-spectrin mutants exhibit a redistribution of beta-Spectrin to the axon scaffold. beta-spectrin mutants show specific dose-dependent genetic interactions with the midline repellent slit and its neuronal receptor roundabout (robo), but not with other guidance molecules. The results suggest that beta-Spectrin contributes to midline repulsion through the regulation of Slit-Robo pathway components. We propose that the Spectrin network is playing a role independently of Ankyrin in the establishment and/or maintenance of specialized membrane domains containing guidance molecules that ensure the fidelity of axon repulsion at the midline.

Son of sevenless directly links the Robo receptor to rac activation to control axon repulsion at the midline

Son of sevenless (Sos) is a dual specificity guanine nucleotide exchange factor (GEF) that regulates both Ras and Rho family GTPases and thus is uniquely poised to integrate signals that affect both gene expression and cytoskeletal reorganization. Here, using genetics, biochemistry, and cell biology, we demonstrate that Sos is recruited to the plasma membrane, where it forms a ternary complex with the Roundabout receptor and the SH3-SH2 adaptor protein Dreadlocks (Dock) to regulate Rac-dependent cytoskeletal rearrangement in response to the Slit ligand. Intriguingly, the Ras and Rac-GEF activities of Sos can be uncoupled during Robo-mediated axon repulsion; Sos axon guidance function depends on its Rac-GEF activity, but not its Ras-GEF activity. These results provide in vivo evidence that the Ras and RhoGEF domains of Sos are separable signaling modules and support a model in which Robo recruits Sos to the membrane via Dock to activate Rac during midline repulsion.

The homeobox transcription factor even-skipped regulates netrin-receptor expression to control dorsal motor-axon projections in Drosophila

Homeobox transcription-factor codes control motor-neuron subtype identity and dorsal versus ventral axon guidance in both vertebrate and invertebrate nervous systems; however, the specific axon guidance-receptors that are regulated by these transcription factors to control pathfinding are poorly defined. In Drosophila, the Even-skipped (Eve) transcription factor specifies dorsal motor-axon projection through the regulation of unidentified guidance molecules. The Netrins and their attractive and repulsive receptors DCC and Unc-5, respectively, define important conserved cue and receptor families that control growth-cone guidance. In Drosophila, the Netrins and frazzled (the fly homolog of DCC) contribute to motor-axon guidance. Here, using genetics and single-cell mRNA-expression analysis, we show that expression and requirement of different Netrin receptor combinations correlate with distinct dorsal and ventral motor-axon projections in Drosophila. Mis-expression of eve dorsalizes ventral axons in part through the upregulation of Unc-5, whereas loss of eve function in two dorsally projecting motor neurons results in aberrant axon projections and a failure to express Unc-5. Our results support a functional link between the expression of distinct Netrin receptor combinations and the transcriptional control of dorsal motor-axon guidance.

Axon guidance at the midline: from mutants to mechanisms

How axons in the developing nervous system successfully navigate to their correct targets is a fundamental problem in neurobiology. Understanding the mechanisms that mediate axon guidance will give important insight into how the nervous system is correctly wired during development and may have implications for therapeutic approaches to developmental brain disorders and nerve regeneration. Achieving this understanding will require unraveling the molecular logic that ensures the proper expression and localization of axon guidance cues and receptors, and elucidating the signaling events that regulate the growth cone cytoskeleton in response to guidance receptor activation. Studies of axon guidance at the midline of many experimental systems, from the ventral midline of Drosophila to the vertebrate spinal cord, have led to important mechanistic insights into the complex problem of wiring the nervous system. Here we review recent advances in understanding the regulation of midline axon guidance, with a particular emphasis on the contributions made from molecular genetic studies of invertebrate model systems.

Cross GTPase-activating protein (CrossGAP)/Vilse links the Roundabout receptor to Rac to regulate midline repulsion

The regulators of the Rho-family GTPases, GTPase-activating proteins (GAPs) and guanine exchange factors (GEFs), play important roles in axon guidance. By means of a functional genomic study of the Rho-family GEFs and GAPs in Drosophila, we have identified a Rho-family GAP, CrossGAP (CrGAP), which is involved in Roundabout (Robo) receptor-mediated repulsive axon guidance. CrGAP physically associates with the Robo receptor. Too much or too little CrGAP activity leads to defects in Robo-mediated repulsion at the midline choice point. The CrGAP gain-of-function phenotype mimics the loss-of-function phenotypes of both Robo and Rac. Dosage-sensitive genetic interactions among CrGAP, Robo, and Rac support a model in which CrGAP transduces signals downstream of Robo receptor to regulate Rac-dependent cytoskeletal changes.

Slit stimulation recruits Dock and Pak to the roundabout receptor and increases Rac activity to regulate axon repulsion at the CNS midline

Drosophila Roundabout (Robo) is the founding member of a conserved family of repulsive axon guidance receptors that respond to secreted Slit proteins. Here we present evidence that the SH3-SH2 adaptor protein Dreadlocks (Dock), the p21-activated serine-threonine kinase (Pak), and the Rac1/Rac2/Mtl small GTPases can function during Robo repulsion. Loss-of-function and genetic interaction experiments suggest that limiting the function of Dock, Pak, or Rac partially disrupts Robo repulsion. In addition, Dock can directly bind to Robo's cytoplasmic domain, and the association of Dock and Robo is enhanced by stimulation with Slit. Furthermore, Slit stimulation can recruit a complex of Dock and Pak to the Robo receptor and trigger an increase in Rac1 activity. These results provide a direct physical link between the Robo receptor and an important cytoskeletal regulatory protein complex and suggest that Rac can function in both attractive and repulsive axon guidance.

A novel Dbl family RhoGEF promotes Rho-dependent axon attraction to the central nervous system midline in Drosophila and overcomes Robo repulsion

The key role of the Rho family GTPases Rac, Rho, and CDC42 in regulating the actin cytoskeleton is well established (Hall, A. 1998. Science. 279:509-514). Increasing evidence suggests that the Rho GTPases and their upstream positive regulators, guanine nucleotide exchange factors (GEFs), also play important roles in the control of growth cone guidance in the developing nervous system (Luo, L. 2000. Nat. Rev. Neurosci. 1:173-180; Dickson, B.J. 2001. Curr. Opin. Neurobiol. 11:103-110). Here, we present the identification and molecular characterization of a novel Dbl family Rho GEF, GEF64C, that promotes axon attraction to the central nervous system midline in the embryonic Drosophila nervous system. In sensitized genetic backgrounds, loss of GEF64C function causes a phenotype where too few axons cross the midline. In contrast, ectopic expression of GEF64C throughout the nervous system results in a phenotype in which far too many axons cross the midline, a phenotype reminiscent of loss of function mutations in the Roundabout (Robo) repulsive guidance receptor. Genetic analysis indicates that GEF64C expression can in fact overcome Robo repulsion. Surprisingly, evidence from genetic, biochemical, and cell culture experiments suggests that the promotion of axon attraction by GEF64C is dependent on the activation of Rho, but not Rac or Cdc42.

Repulsive axon guidance: Abelson and Enabled play opposing roles downstream of the roundabout receptor

Drosophila Roundabout (Robo) is the founding member of a conserved family of repulsive axon guidance receptors that respond to secreted Slit proteins. Little is known about the signaling mechanisms which function downstream of Robo to mediate repulsion. Here, we present genetic and biochemical evidence that the Abelson (Abl) tyrosine kinase and its substrate Enabled (Ena) play direct and opposing roles in Robo signal transduction. Genetic interactions support a model in which Abl functions to antagonize Robo signaling, while Ena is required in part for Robo's repulsive output. Both Abl and Ena can directly bind to Robo's cytoplasmic domain. A mutant form of Robo that interferes with Ena binding is partially impaired in Robo function, while a mutation in a conserved cytoplasmic tyrosine that can be phosphorylated by Abl generates a hyperactive Robo receptor.

Chimeric axon guidance receptors: the cytoplasmic domains of slit and netrin receptors specify attraction versus repulsion

Frazzled (Fra) is the DCC-like Netrin receptor in Drosophila that mediates attraction; Roundabout (Robo) is a Slit receptor that mediates repulsion. Both ligands are expressed at the midline; both receptors have related structures and are often expressed by the same neurons. To determine if attraction versus repulsion is a modular function encoded in the cytoplasmic domain of these receptors, we created chimeras carrying the ectodomain of one receptor and the cytoplasmic domain of the other and tested their function in transgenic Drosophila. Fra-Robo (Fra's ectodomain and Robo's cytoplasmic domain) functions as a repulsive Netrin receptor; neurons expressing Fra-Robo avoid the Netrin-expressing midline and muscles. Robo-Fra (Robo's ectodomain and Fra's cytoplasmic domain) is an attractive Slit receptor; neurons and muscle precursors expressing Robo-Fra are attracted to the Slit-expressing midline.

The regulation of the Drosophila msl-2 gene reveals a function for Sex-lethal in translational control

In Drosophila, dosage compensation occurs by increasing the transcription of the single male X chromosome. Four trans-acting factors encoded by the male-specific lethal genes are required for this process. Dosage compensation is restricted to males by the splicing regulator Sex-lethal, which functions to prevent the production of the MSL-2 protein in females by an unknown mechanism. In this report, we provide evidence that Sex-lethal acts synergistically through sequences in both the 5' and 3' untranslated regions of MSL-2 to mediate repression. We also provide evidence that the repression of MSL-2 is directly regulated by Sex-lethal at the level of translation.

The dosage compensation system of Drosophila is co-opted by newly evolved X chromosomes

In species where males and females differ in number of sex chromosomes, the expression of sex-linked genes is equalized by a process known as dosage compensation. In Drosophila melanogaster, dosage compensation is mediated by the binding of the products of the male-specific lethal (msl) genes to the single male X chromosome. Here we report that the sex- and chromosome-specific binding of three of the msl proteins (MSLs) occurs in other drosophilid species, spanning four genera. Moreover, we show that MSL binding correlates with the evolution of the sex chromosomes: in species that have acquired a second X chromosome arm because of an X-autosome translocation, we observe binding of the MSLs to the 'new' (previously autosomal) arm of the X chromosome, only when its homologue has degenerated. Moreover, in Drosophila miranda, a Y-autosome translocation has produced a new X chromosome (called neo-X), only some regions of which are dosage compensated. In this neo-X chromosome, the pattern of MSL binding correlates with the known pattern of dosage compensation.

Evidence that MSL-mediated dosage compensation in Drosophila begins at blastoderm

In Drosophila equalization of the amounts of gene products produced by X-linked genes in the two sexes is achieved by hypertranscription of the single male X chromosome. This process, dosage compensation, is controlled by a set of male-specific lethal (msl) genes, that appear to act at the level of chromatin structure. The properties of the MSL proteins have been extensively studied in the polytene salivary gland chromosomes where they bind to the same set of sites along the male X chromosome in a co-dependent manner. Here we report experiments that show that the MSL proteins first associate with the male X chromosome as early as blastoderm stage, slightly earlier than the histone H4 isoform acetylated at lysine 16 is detected on the X chromosome. MSL binding to the male X chromosome is observed in all somatic tissues of embryos and larvae. Binding of the MSLs to the X chromosome is also interdependent in male embryos and prevented in female embryos by the expression of Sex-lethal (Sxl). A delayed onset of binding of the MSLs in male progeny of homozygous mutant msl-1 or mle mothers coupled with the previous finding that such males have an earlier lethal phase supports the idea that msl-mediated dosage compensation begins early in embryogenesis. Other results show that the maleless (MLE) protein on embryo and larval chromosomes differs in its reactivity with antibodies; the functional significance of this finding remains to be explored.

Dosage compensation and chromatin structure in Drosophila

The past year has brought significant advances in our understanding of the male-specific lethal (msl genes and dosage compensation in Drosophila. The molecular characterization of the msl-2 gene has, to a great extent, solved the question of how msl-mediated dosage compensation is restricted to males. Molecular analyses of the msl genes have substantiated the proposal that the MSL proteins function as a multimeric complex to mediate dosage compensation. The finding that MSL-2 protein has a RING finger and the demonstration that an insulator protein facilitates the dosage compensation of X-linked genes inserted into the autosomes have opened promising avenues to identify the cis-acting dosage-compensation determinants.

The msl-2 dosage compensation gene of Drosophila encodes a putative DNA-binding protein whose expression is sex specifically regulated by Sex-lethal

In Drosophila dosage compensation increases the rate of transcription of the male's X chromosome and depends on four autosomal male-specific lethal genes. We have cloned the msl-2 gene and shown that MSL-2 protein is co-localized with the other three MSL proteins at hundreds of sites along the male polytene X chromosome and that this binding requires the other three MSL proteins. msl-2 encodes a protein with a putative DNA-binding domain: the RING finger. MSL-2 protein is not produced in females and sequences in both the 5′ and 3′ UTRs are important for this sex-specific regulation. Furthermore, msl-2 pre-mRNA is alternatively spliced in a Sex-lethal-dependent fashion in its 5′ UTR.